1. Field of the Invention
The present invention relates to a power transmission apparatus, and more specifically, to a power transmission apparatus that has an electromagnetic friction clutch and a method for tuning the power transmission apparatus.
2. Discussion of the Background
Japanese Laid-Open Patent Publication No. 11-153159 discloses a conventional power transmission apparatus. The power transmission apparatus includes inner and outer rotary members, which are coaxially arranged with and rotate relative to each other, an annular friction clutch, which is located between the inner rotary member and the outer rotary member, and an electromagnetic drive apparatus, which drives the friction clutch when energized by current supply.
The electromagnetic drive apparatus has an annular armature, which is arranged to face the friction clutch, and an annular electromagnet, which is arranged to face the friction clutch with part of the outer rotary member located in between. When the electromagnet is energized, the armature is attracted to the electromagnet and causes friction with the friction clutch. Thus, the inner and outer rotary members are coupled with each other via the friction clutch to transmit torque.
The electromagnet is supported by an annular electromagnet support formed on a yoke. The electromagnet support and the electromagnet are accommodated in an annular groove formed at part of the outer rotary member. Predetermined outer and inner clearances are located between the electromagnet support and the annular groove. More specifically, the outer circumferential surface of the electromagnet support is apart from one side of the annular groove by a predetermined first distance. The inner circumferential surface of the electromagnet support is apart from the other side of the annular groove by a predetermined second distance.
When exciting current is supplied to the electromagnet, a magnetic path that circulates the yoke, the outer clearance, part of the outer rotary member, the friction clutch, the armature, the friction clutch, part of the outer rotary member, the inner clearance, and the yoke is generated. The armature is attracted toward the electromagnet by the operation of the magnetic induction and depresses the friction clutch. Thus, the outer rotary member and the inner rotary member are coupled to each other to transmit torque.
In an assembling procedure of the power transmission apparatus, the inner rotary member, the armature, and the friction clutch are mounted inside the outer rotary member. The yoke to which the electromagnet is attached is assembled in the last process.
In the assembling procedure of the yoke, an operator selects one of yokes having several sizes and mounts the yoke to the outer rotary member. Then, a conduction test of the electromagnet is performed to measure the exciting current/transmission torque characteristic. If the characteristic is undesirable, the operator changes the yoke with other yoke having different size that provides satisfactory exciting current/transmission torque characteristic.
The exciting current/transmission torque characteristic refers to the relationship between the exciting current supplied to the electromagnet (electromagnetic coil) and the torque transmitted from the outer rotary member to the inner rotary member.
Each yoke is formed such that the outer clearance and the inner clearance will be equal. More specifically, the inner circumferential surface and the outer circumferential surface of each yoke are cut such that the first and the second distances will be equal. Therefore, the manufacturing process of the yokes of the conventional power transmission apparatus is troublesome. Also, the tuning variation of the exciting current/transmission torque characteristic is relatively large.